1. Field of the Invention
The present invention relates generally to an improved dielectric lubricant and more particularly to the use of the improved lubricant in a spark plug boot.
2. Description of the Prior Art
Spark plug boots and the insulation on the wires associated therewith are almost universally made with silicone rubbers. The spark plug boot has a cavity formed therein which is adapted for receiving a spark plug. Various agents have been employed at the spark plug-to-spark plug boot interface for the purposes of improving the dielectric capability of the interface and for preventing the spark plug and spark plug boot from bonding to one another.
U.S. Pat. No. 5,385,686, Miller et al., discloses a spark plug boot lubricant which includes a perfuoroalkylpolyether in combination with an extender in the form of a silicone grease, such as polydimethylsiloxane. Various fillers, such as silica, are also suggested.
Perfuoroalkylpolyether lubricants with polydimethylsiloxane extenders such as those disclosed in Miller et al. have been found to suffer from several drawbacks. Polydimethylsiloxane-containing lubricants often migrate into the silicone rubber of the spark plug boot, causing deterioration. Furthermore, the silicone-based lubricants of the prior art frequently do not tolerate the high temperature conditions to which the spark plug boot and lubricants are exposed. Lower molecular weight components of the silicone can bleed and/or evaporate, thus leaving behind only a gummy residue. In addition to the loss of dielectric properties, this can result in the spark plug boot bonding to the spark plug, making removal of the boot difficult. The bonding can be so severe that the spark plug boot undergoes mechanical failure upon removal.
Thus, there exists a need for a spark plug boot lubricant that can withstand a high temperature environment without the loss of dielectric properties, which does not cause physical deterioration of the spark plug boot and which prevents the spark plug boot from bonding to the spark plug.
The present inventors have discovered a novel spark plug boot lubricant that withstands the high temperature environment to which spark plug boots are subjected, without causing deterioration of the silicone rubber of the spark plug boot. Furthermore, the lubricant of the present invention prevents the spark plug boot from bonding to the spark plug and maintains its dielectric properties during use. The present invention is further directed to an improved spark plug boot in combination with the lubricant of the invention.
In accordance with the invention, there is provided a spark plug boot lubricant comprising:
(A) a phenyl-methyl siloxane having an average general formula
Me3SiOxe2x80x94(Me2SiO)x(MePhSiO)yxe2x80x94SiMe3
wherein each Me represents a methyl group, each Ph represents a phenyl group, x has a value from zero to about 0.25 and x+y=1;
said phenyl-methyl siloxane having a viscosity at 25xc2x0 C. from about 500 to about 10,000 centipoise;
(B) a polytetrafluoroethylene powdered solid; and
component (A) being present in an amount of 50 to 80 parts by weight and component (B) being present in an amount of 20 to 50 parts, by weight, per 100 parts by weight of the combined weights of (A) and (B).
Further in accordance with the invention, there is provided an improved silicone rubber spark plug boot. The spark plug boot has a spark plug cavity formed therein, the cavity being defined by cavity walls. The improvement comprises the cavity walls having the above-described phenyl-methyl siloxane lubricant applied thereto.